Acoustic filter employing inductive coupling

ABSTRACT

Disclosed in one embodiment is filter circuitry having first and second paths extending between first and second nodes. The first path has a first inductor and a second inductor coupled in series between the first node and the second node, wherein the first inductor and the second inductor are positively coupled with one another, and a first common node is provided between the first inductor and the second inductor. First shunt acoustic resonators are coupled between the first common node and a fixed voltage node. The second path includes a third inductor and a fourth inductor coupled in series between the first node and the second node. The third inductor and the fourth inductor are negatively coupled with one another, and a second common node is provided between the third inductor and the fourth inductor. Second acoustic resonators are coupled between the second common node and a fixed voltage node.

RELATED APPLICATIONS

This application claims the benefit of provisional patent application Ser. No. 62/394,824, filed Sep. 15, 2016, the disclosure of which is hereby incorporated herein by reference in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to acoustic filters that employ inductive coupling.

BACKGROUND

Acoustic resonators, such as Surface Acoustic Wave (SAW) resonators and Bulk Acoustic Wave (BAW) resonators, are used in many high-frequency communication applications. In particular, SAW resonators are often employed in filter networks that operate at frequencies up to 1.8 GHz, and BAW resonators are often employed in filter networks that operate at frequencies above 1.5 GHz. Such filters need to have flat passbands, have steep filter skirts and squared shoulders at the upper and lower ends of the passband, and provide excellent rejection outside of the passband. SAW- and BAW-based filters also have relatively low insertion loss, tend to decrease in size as the frequency of operation increases, and are relatively stable over wide temperature ranges.

As such, SAW- and BAW-based filters are the filter of choice for many 3rd Generation (3G) and 4th Generation (4G) wireless devices and are destined to dominate filter applications for 5th Generation (5G) wireless devices. Most of these wireless devices support cellular, Wireless Fidelity (WiFi), Bluetooth, and/or near field communications on the same wireless device and, as such, pose extremely challenging filtering demands. While these demands keep raising the complexity of wireless devices, there is a constant need to improve the performance of acoustic resonators and filters that are based thereon.

To better understand acoustic resonators and the various terminology associated therewith, the following provides an overview of a BAW resonator. However, the concepts described herein may employ any type of acoustic resonator, such as SAW-based resonators, and are not limited to BAW-based resonators. An exemplary BAW resonator 10 is illustrated in FIG. 1. The BAW resonator 10 generally includes a substrate 12, a reflector 14 mounted over the substrate 12, and a transducer 16 mounted over the reflector 14. The transducer 16 rests on the reflector 14 and includes a piezoelectric layer 18, which is sandwiched between a top electrode 20 and a bottom electrode 22. The top and bottom electrodes 20 and 22 may be formed of Tungsten (W), Molybdenum (Mo), Platinum (Pt), or like material, and the piezoelectric layer 18 may be formed of Aluminum Nitride (AlN), Zinc Oxide (ZnO), or other appropriate piezoelectric material. Although shown in FIG. 1 as each including a single layer, the piezoelectric layer 18, the top electrode 20, and/or the bottom electrode 22 may include multiple layers of the same material, multiple layers in which at least two layers are different materials, or multiple layers in which each layer is a different material.

The BAW resonator 10 is divided into an active region 24 and an outside region 26. The active region 24 generally corresponds to the section of the BAW resonator 10 where the top and bottom electrodes 20 and 22 overlap and also includes the layers below the overlapping top and bottom electrodes 20 and 22. The outside region 26 corresponds to the section of the BAW resonator 10 that surrounds the active region 24.

For the BAW resonator 10, applying electrical signals across the top electrode 20 and the bottom electrode 22 excites acoustic waves in the piezoelectric layer 18. These acoustic waves primarily propagate vertically. A primary goal in BAW resonator design is to confine these vertically propagating acoustic waves in the transducer 16. Acoustic waves traveling upward are reflected back into the transducer 16 by the air-metal boundary at the top surface of the top electrode 20. Acoustic waves traveling downward are reflected back into the transducer 16 by the reflector 14 or by an air cavity, which is provided just below the transducer in a Film BAW Resonator.

The reflector 14 is typically formed by a stack of reflector layers (RL) 28, which alternate in material composition to produce a significant reflection coefficient at the junction of adjacent reflector layers 28. Typically, the reflector layers 28 alternate between materials having high and low acoustic impedances, such as Tungsten (W) and Silicon Dioxide (SiO₂). While only five reflector layers 28 are illustrated in FIG. 1, the number of reflector layers 28 and the structure of the reflector 14 varies from one design to another.

The magnitude (Z) and phase (ϕ) of the electrical impedance as a function of the frequency for a relatively ideal BAW resonator 10 is provided in FIG. 2. The magnitude (Z) of the electrical impedance is illustrated by the solid line, whereas the phase (ϕ) of the electrical impedance is illustrated by the dashed line. A unique feature of the BAW resonator 10 is that it has both a resonance frequency and an anti-resonance frequency. The resonance frequency is typically referred to as the series resonance frequency (f_(s)), and the anti-resonance frequency is typically referred to as the parallel resonance frequency (f_(p)). The series resonance frequency (f_(s)) occurs when the magnitude of the impedance, or reactance, of the BAW resonator 10 approaches zero. The parallel resonance frequency (f_(p)) occurs when the magnitude of the impedance, or reactance, of the BAW resonator 10 peaks at a significantly high level. In general, the series resonance frequency (f_(s)) is a function of the thickness of the piezoelectric layer 18 and the mass of the bottom and top electrodes 20 and 22.

For the phase, the BAW resonator 10 acts like an inductance that provides a 90° phase shift between the series resonance frequency (f_(s)) and the parallel resonance frequency (f_(p)). In contrast, the BAW resonator 10 acts like a capacitance that provides a −90° phase shift below the series resonance frequency (f_(s)) and above the parallel resonance frequency (f_(p)). The BAW resonator 10 presents a very low, near zero, resistance at the series resonance frequency (f_(s)) and a very high resistance at the parallel resonance frequency (f_(p)). The electrical nature of the BAW resonator 10 lends itself to the realization of a very high Q (quality factor) inductance over a relatively short range of frequencies, which has proved to be very beneficial in high-frequency filter networks, especially those operating at frequencies around 1.8 GHz and above.

Unfortunately, the phase (ϕ) curve of FIG. 2 is representative of an ideal phase curve. In reality, approaching this ideal is challenging. A typical phase curve for the BAW resonator 10 of FIG. 1 is illustrated in FIG. 3A. Instead of being a smooth curve, the phase curve of FIG. 3A includes ripple below the series resonance frequency (f_(s)), between the series resonance frequency (f_(s)) and the parallel resonance frequency (f_(p)), and above the parallel resonance frequency (f_(p)). The ripple is the result of spurious modes, which are caused by spurious resonances that occur in corresponding frequencies. While the vast majority of the acoustic waves in the BAW resonator 10 propagate vertically, various boundary conditions about the transducer 16 result in the propagation of lateral (horizontal) acoustic waves, which are referred to as lateral standing waves. The presence of these lateral standing waves reduces the potential Q associated with the BAW resonator 10.

As illustrated in FIG. 4, a border (BO) ring 30 is formed on or within the top electrode 20 to suppress certain of the spurious modes. The spurious modes that are suppressed by the BO ring 30 are those above the series resonance frequency (f_(s)), as highlighted by circles A and B in the phase curve of FIG. 3B. Circle A shows a suppression of the ripple, and thus of the spurious mode, in the passband of the phase curve, which resides between the series resonance frequency (f_(s)) and the parallel resonance frequency (f_(p)). Circle B shows suppression of the ripple, and thus of the spurious modes, above the parallel resonance frequency (f_(p)). Notably, the spurious mode in the upper shoulder of the passband, which is just below the parallel resonance frequency f_(p), and the spurious modes above the passband are suppressed, as evidenced by the smooth or substantially ripple free phase curve between the series resonance frequency (f_(s)) and the parallel resonance frequency (f_(p)) and above the parallel resonance frequency (f_(p)).

The BO ring 30 corresponds to a mass loading of the portion of the top electrode 20 that extends about the periphery of the active region 24. The BO ring 30 may correspond to a thickened portion of the top electrode 20 or the application of additional layers of an appropriate material over the top electrode 20. The portion of the BAW resonator 10 that includes and resides below the BO ring 30 is referred to as a BO region 32. Accordingly, the BO region 32 corresponds to an outer, perimeter portion of the active region 24 and resides inside of the active region 24.

While the BO ring 30 is effective at suppressing spurious modes above the series resonance frequency (f_(s)), the BO ring 30 has little or no impact on those spurious modes below the series resonance frequency (f_(s)), as shown by the ripples in the phase curve below the series resonance frequency (f_(s)) in FIG. 3B. A technique referred to as apodization is often used to suppress the spurious modes that fall below the series resonance frequency (f_(s)).

Apodization tries to avoid, or at least significantly reduce, any lateral symmetry in the BAW resonator 10, or at least in the transducer 16 thereof. The lateral symmetry corresponds to the footprint of the transducer 16, and avoiding the lateral symmetry corresponds to avoiding symmetry associated with the sides of the footprint. For example, one may choose a footprint that corresponds to a pentagon instead of a square or rectangle. Avoiding symmetry helps reduce the presence of lateral standing waves in the transducer 16. Circle C of FIG. 3C illustrates the effect of apodization in which the spurious modes below the series resonance frequency (f_(s)) are suppressed, as evidence by the smooth or substantially ripple free phase curve below the series resonance frequency (f_(s)). Assuming no BO ring 30 is provided, one can readily see in FIG. 3C that apodization fails to suppress those spurious modes above the series resonance frequency (f_(s)). As such, the typical BAW resonator 10 employs both apodization and the BO ring 30.

As noted previously, BAW resonators 10 are often used in filter networks that operate at high frequencies and require high Q values. A basic ladder network 40 is illustrated in FIG. 5A. The ladder network 40 includes two series resonators B_(SER) and two shunt resonators B_(SH), which are arranged in a traditional ladder configuration. Typically, the series resonators B_(SER) have the same or similar first frequency response, and the shunt resonators B_(SH) have the same or similar second frequency response, which is different from the first frequency response, as shown in FIG. 5B. In many applications, the shunt resonators B_(SH) are detuned versions of the series resonators B_(SER). As a result, the frequency responses for the series resonators B_(SER) and the shunt resonators B_(SH) are generally very similar, yet shifted relative to one another such that the parallel resonance frequency (f_(p,SH)) of the shunt resonators approximates the series resonance frequency (f_(s,SER)) of the series resonators B_(SER). Note that the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) is less than the series resonance frequency (f_(s,SER)) of the series resonators B_(SER). The parallel resonance frequency (f_(p,SH)) of the shunt resonators B_(SH) is less than the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER).

FIG. 5C is associated with FIG. 5B and illustrates the response of the ladder network 40. The series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) corresponds to the low side of the passband's skirt (phase 2), and the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER) corresponds to the high side of the passband's skirt (phase 4). The substantially aligned series resonance frequency (f_(s,SER)) of the series resonators B_(SER) and the parallel resonance frequency (f_(p,SH)) of the shunt resonators B_(SH) fall within the passband. FIGS. 6A through 6E provide circuit equivalents for the five phases of the response of the ladder network 40. During the first phase (phase 1, FIGS. 5C, 6A), the ladder network 40 functions to attenuate the input signal. As the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) is approached, the impedance of the shunt resonators B_(SH) drops precipitously such that the shunt resonators B_(SH) essentially provide a short to ground at the series resonance frequency (f_(s,SH)) of the shunt resonators (phase 2, FIGS. 5C, 6B). At the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) (phase 2), the input signal is essentially blocked from the output of the ladder network 40.

Between the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) and the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER), which corresponds to the passband, the input signal is passed to the output with relatively little or no attenuation (phase 3, FIGS. 5C, 6C). Within the passband, the series resonators B_(SER) present relatively low impedance, whereas the shunt resonators B_(SH) present relatively high impedance, wherein the combination of the two leads to a flat passband with steep low- and high-side skirts. As the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER) is approached, the impedance of the series resonators B_(SER) becomes very high, such that the series resonators B_(SER) essentially present themselves as open at the parallel resonance frequency (f_(p,SER)) of the series resonators (phase 4, FIGS. 5C, 6D). At the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER) (phase 4), the input signal is again essentially blocked from the output of the ladder network 40.

During the final phase (phase 5, FIGS. 5C, 6E), the ladder network 40 functions to attenuate the input signal, in a similar fashion to that provided in phase 1. As the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER) is passed, the impedance of the series resonators B_(SER) decreases and the impedance of the shunt resonators B_(SH) normalizes. Thus, the ladder network 40 functions to provide a high Q passband between the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) and the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER). The ladder network 40 provides extremely high attenuation at both the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) and the parallel resonance frequency (f_(p,SER)) of the series resonators. The ladder network 40 provides good attenuation below the series resonance frequency (f_(s,SH)) of the shunt resonators B_(SH) and above the parallel resonance frequency (f_(p,SER)) of the series resonators B_(SER). As noted previously, there is a constant need to improve the performance of acoustic resonators and filters that are based thereon.

SUMMARY

Disclosed in one embodiment is filter circuitry having first and second paths extending between first and second nodes. The first path has a first inductor and a second inductor coupled in series between the first node and the second node, wherein the first inductor and the second inductor are positively coupled with one another, and a first common node is provided between the first inductor and the second inductor. First shunt acoustic resonators are coupled between the first common node and a fixed voltage node. The second path includes a third inductor and a fourth inductor coupled in series between the first node and the second node. The third inductor and the fourth inductor are negatively coupled with one another, and a second common node is provided between the third inductor and the fourth inductor. Second acoustic resonators are coupled between the second common node and a fixed voltage node. The filter response of the filter circuitry is one of a passband, a stopband, or a multiple passband/stopband provided between the first node and the second node.

Disclosed in another embodiment is filter circuitry having an input node, a first node, a second node, an output node, and a fixed voltage node. First acoustic resonators are coupled in parallel with one another between the input node and the first node. A first inductor is coupled between the first node and the fixed voltage node, such that the first acoustic resonators are coupled in series with the first inductor between the input node and the fixed voltage node. Second acoustic resonators are coupled in parallel with one another between the second node and the output node. A second inductor is coupled between the second node and the fixed voltage node, such that the second acoustic resonators are coupled in series with the second inductor between the output node and the fixed voltage node, wherein the first inductor and the second inductor are negatively coupled with one another.

The filter circuit may also have a third node and a fourth node, wherein third acoustic resonators are coupled in parallel with one another between the input node and the first node. A third inductor is coupled between the third node and the fixed voltage node, such that the third plurality of acoustic resonators is coupled in series with the third inductor between the input node and the fixed voltage node. Fourth acoustic resonators are coupled in parallel with one another between the fourth node and the output node. A fourth inductor is coupled between the fourth node and the fixed voltage node, such that the fourth acoustic resonators are coupled in series with the fourth inductor between the output node and the fixed voltage node, wherein the third inductor and the fourth inductor are positively coupled with one another.

A differential power amplifier circuit is provided in yet another embodiment. The differential power amplifier circuit includes a first power amplifier having a first input node and a first output node and a second power amplifier having a second input node and a second output node. A first transformer includes a primary side coupled between the first output node and the second output node and a secondary side formed from a first inductor and a second inductor coupled in series between a first node and a second node, wherein a common node is provided between the first inductor and the second inductor. At least one series acoustic resonator is coupled between the first node and the second node, wherein at least one main series resonance is provided between the first node and the second node at a main resonance frequency through the at least one series acoustic resonator. A first shunt acoustic resonator is coupled between the common node and a fixed voltage node. A second shunt acoustic resonator is coupled between the common node and the fixed voltage node, wherein a differential input is provided across the first input node and the second input node, and a single-ended output signal is provided at the second node.

Those skilled in the art will appreciate the scope of the present disclosure and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the disclosure and, together with the description, serve to explain the principles of the disclosure.

FIG. 1 illustrates a conventional Bulk Acoustic Wave (BAW) resonator.

FIG. 2 is a graph of the magnitude and phase of impedance over frequency responses as a function of frequency for an ideal BAW resonator.

FIGS. 3A-3C are graphs of phase responses for various BAW resonator configurations.

FIG. 4 illustrates a conventional BAW resonator with a border ring.

FIG. 5A is a schematic of a conventional ladder network.

FIGS. 5B and 5C are graphs of a frequency response for BAW resonators in the conventional ladder network of FIG. 5A and a frequency response for the conventional ladder network of FIG. 5A.

FIGS. 6A-6E are circuit equivalents for the ladder network of FIG. 5A at the frequency points 1, 2, 3, 4, and 5, which are identified in FIG. 5C.

FIG. 7 is a block diagram of a mobile terminal according to one embodiment.

FIG. 8 is a schematic of an RF front-end according to a first embodiment.

FIG. 9 illustrates a rigid PCB and a flexible PCB coupled together by multiple coaxial cables according to a first embodiment.

FIG. 10 is a schematic of an RF front-end according to a second embodiment.

FIG. 11 illustrates an acoustic resonator in parallel with a compensation circuit, which includes a single shunt acoustic resonator.

FIG. 12 is a graph that illustrates exemplary frequency responses for the acoustic resonator, compensation circuit, and overall circuit of FIG. 11.

FIG. 13 illustrates an acoustic resonator in parallel with a compensation circuit, which includes at least two shunt acoustic resonators, according to a first embodiment.

FIG. 14 is a graph that illustrates exemplary frequency responses for the acoustic resonator, compensation circuit, and overall circuit of FIG. 13.

FIG. 15 is a graph that compares actual frequency responses of the overall circuits of FIGS. 11 and 13.

FIG. 16 illustrates a plurality of parallel acoustic resonators in parallel with a compensation circuit, which includes at least two shunt acoustic resonators, according to a second embodiment.

FIG. 17 is a graph that illustrates first exemplary frequency responses for the acoustic resonator, compensation circuit, and overall circuit of FIG. 16.

FIG. 18 is a graph that illustrates second exemplary frequency responses for the acoustic resonator, compensation circuit, and overall circuit of FIG. 16.

FIGS. 19A through 19D illustrate transformation of the T-circuit impedance architecture of the compensation circuit of FIG. 13 to a π (pi) impedance model.

FIG. 20 illustrates the overall circuit of FIG. 13 using the π (pi) impedance model of FIG. 19D.

FIG. 21 is a graph illustrating the overall shunt impedance, Zres, according to one embodiment.

FIG. 22 is a graph illustrating the series equivalent impedance, ZA, according to one embodiment.

FIGS. 23A and 23B are graphs over different frequency ranges illustrating the absolute or magnitude of series impedance ZS, the series equivalent impedance ZA, and overall series impedance ZAs, according to one embodiment.

FIG. 24 is an alternative filter configuration according to one embodiment.

FIG. 25 is an alternative filter configuration according to one embodiment.

FIG. 26 is an alternative filter configuration according to one embodiment.

FIG. 27 is an alternative filter configuration according to one embodiment.

FIG. 28 is an alternative filter configuration according to one embodiment.

FIG. 29 is differential amplifier circuit that employs the concepts provided herein.

FIG. 30 illustrates front end circuitry for a communication device that employs the concepts provided herein according to a first embodiment.

FIG. 31 illustrates front end circuitry for a communication device that employs the concepts provided herein according to a first embodiment.

DETAILED DESCRIPTION

The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the embodiments and illustrate the best mode of practicing the embodiments. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the disclosure and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.

It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and similarly, a second element could be termed a first element, without departing from the scope of the present disclosure. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

It will be understood that when an element such as a layer, region, or substrate is referred to as being “on” or extending “onto” another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or extending “directly onto” another element, there are no intervening elements present. Likewise, it will be understood that when an element such as a layer, region, or substrate is referred to as being “over” or extending “over” another element, it can be directly over or extend directly over the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly over” or extending “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present.

Relative terms such as “below” or “above” or “upper” or “lower” or “horizontal” or “vertical” may be used herein to describe a relationship of one element, layer, or region to another element, layer, or region as illustrated in the figures. It will be understood that these terms and those discussed previously are intended to encompass different orientations of the device in addition to the orientation depicted in the figures.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including” when used herein specify the presence of stated features, integers, steps, operations, elements, and/or components but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

As defined herein, the term “coupled” without being preceded with the adjective “acoustically” refers to an electrical coupling as opposed to an acoustic coupling. The term “acoustically coupled” refers to an acoustic coupling as opposed to an electrical coupling. Further, the phrase “about the same as” when referring to the series resonance frequency of two or more devices means that the series resonance frequencies of the devices are within 0.1% of each other.

While the concepts provided herein are applicable to various technologies, these concepts are particularly useful in mobile terminals, such as mobile telephones, tablets, computers, and like smart devices. The following provides an overview of such devices. Today's mobile terminals must communicate using different communication technologies in different bands, which vary significantly in both bandwidth and frequency. To further complicate matters, data rates are ever increasing and the there is a need to transmit and receive over these different bands at the same time. As a result, mobile terminals have very complicated front-end configurations and are starting to employ multiple input, multiple output (MIMO) transmission and reception technology, which requires the use of multiple antennas.

FIG. 7 is a block diagram of a mobile terminal 42 that incorporates four antennas: a primary antenna A1, a secondary antenna A2, a tertiary antenna A3, and a quaternary antenna A4. The mobile terminal 42 generally includes control circuitry 44, which is associated with a user interface (I/F) 46, and radio frequency (RF) circuitry 48. The user interface 46 may include microphones, speakers, keypads, touchscreens, displays, and the like. The RF circuitry 48 may include baseband, transceiver, power amplifier, and switching circuitry, as will be appreciated by those skilled in the art.

In general, signals to be transmitted are provided by the RF circuitry 48 to one or more of the antennas A1 through A4, and signals received by one or more of the antennas A1 through A4 are routed to the RF circuitry 48 for demodulation and associated processing. The RF circuitry 48 may be configured to facilitate any number of communications, including first-, second-, third-, fourth-, and fifth-generation cellular communications; wireless local area network communications; Bluetooth communications; industrial, scientific, and medical communications; near field communications; and the like. Any of these communications may use MIMO for transmission, reception, or both, depending on the capabilities of the mobile terminal 42 and the systems with which the mobile terminal 42 communicates.

Since mobile terminals 42 are relatively small, the multiple antennas A1 through A4 used for MIMO are relatively close to one another. As a result, the antennas A1 through A4 may interact with one another, and as a result, modify each other's radiation patterns, which generally alters the antenna's radiation efficiency. With continued reference to FIG. 7, when the primary antenna A1 is used for transmission and the tertiary antenna A3 is used for reception at the same time, the transmission from antennae A1 may significantly degrade the ability to receive signals via antenna A3, given the proximity of antenna A1 to antenna A3. Further, the secondary antenna A2 and the quaternary antenna A4 may also be affected by transmissions from the primary antenna A1. As such, there is a need for a cost-effective and space-efficient technique to resolve, or at least significantly reduce, the impact that one antenna has on another in devices such as the mobile terminal 42 illustrated in FIG. 7.

With reference to FIG. 8, a technique for addressing the foregoing issues is described. As illustrated, the RF circuitry 48 is associated with antennas A1 through A4. Each of the antennas is associated with antenna tuning circuitry 50, 54, 52, 56, respectively. In particular, antenna A1 is coupled to the RF circuitry 48 through coaxial cable 58. Antenna A3 is coupled to the RF circuitry 48 through coaxial cable 60 and a MPMS filter 62; antenna A2 is coupled to the RF circuitry 48 through coaxial cable 64 and MPMS filter 66; and antenna A4 is coupled to the RF circuitry 48 through coaxial cable 68 and MPMS filter 70. While MPMS filters 62, 66, and 70 are provided for antennas A3, A2, and A4, respectively, alternative embodiments may only employ MPMS filter 62, given the proximity of antennas A1 and A3. In other embodiments, an appropriately configured MPMS filter (not illustrated) may also be provided in association with antenna A1. In short, MPMS filters may be provided for each antenna A1 through A4 or any combination thereof. The antenna tuning circuitry 50, 52, 54, 56 are used for tuning impedances associated with the respective antennas A1, A3, A2, A4, as those skilled in the art will appreciate.

For the following description, MPMS filter 62 is described in detail; however, MPMS filter 66 and 70 may be similarly or identically configured, depending on the embodiment. Assume that the RF circuitry 48 is configured to transmit RF signals in band X and band Y via antenna A1 at the same or different times. Further assume that RF circuitry 48 is configured to receive RF signals and bands A, B, and C at the same or different times. Given the proximity of antennas A1 and A3, transmitting in bands X or Y via antenna A1 would significantly affect the ability of antenna A3 to receive RF signals in bands A, B, or C, in the absence of MPMS filter 62. However, adding MPMS filter 62 in proximity to antenna A3 significantly reduces the impact that antenna A1 has on antenna A3.

MPMS filter 62 is a specially configured filter that has multiple passbands and multiple stopbands, which are interleaved with one another, as illustrated in FIG. 8. In this example, passbands are provided for bands A, B, and C and stopbands are provided for at least bands X and Y. A stopband for band X is between passbands for bands A and B, and a stopband for band Y is between passbands for bands B and C. In other words, stopbands are provided for the problematic bands that are transmitted via antenna A1, and passbands are provided for the bands to be received via antenna A3. The RF circuitry 48 may also transmit signals in bands A, B, or C via antenna A3. Providing stopbands for an adjacent antenna's transmission bands and passbands for the selected antenna's receive (and transmission) bands can significantly improve the performance of both antennas. When multiple ones of the MPMS filters 62, 66, 70 are employed, the passbands and stopbands may be the same or different among the different MPMS filters 62, 66, 70, based on the proximity of the antennas A1-A4 and the communication bands used for communications by the mobile terminal 42.

In certain embodiments, at least two of the stopbands and/or passbands provided by one or more of the MPMS filters 62, 66, 70 reside entirely above 2 GHz and have a bandwidth of at least 20 MHz. In other embodiments, at least two of the stopbands and/or passbands reside between 2 GHz and 12 GHz and have a bandwidth of at least 20 MHz, 40 MHz, 50 MHz, or 100 MHz. In select embodiments, at least one of the stopbands or passbands residing between two other stopbands or passbands has a bandwidth of at least 100 MHz, 150 MHz, or 200 MHz. All, or at least certain of, the stopbands may provide attenuation of at least 10 dB, 20 dB, or 30 dB in each of the foregoing embodiments, depending on the configuration of the MPMS filters 62, 66, 70.

With reference to FIG. 9, the mobile terminal 42 may employ multiple printed circuit boards (PCBs) to implement the necessary electronics for operation. Further, the various antennas A1-A4 may be spread about the mobile terminal 42. These antennas A1-A4 may be implemented on or in a housing H (illustrated in FIG. 7) of the mobile terminal 42, on the various PCBs, or a combination thereof. FIG. 9 illustrates a rigid PCB (R-PCB) and a flexible PCB (F-PCB), which are used to implement at least part of the electronics for the mobile terminal 42. In one embodiment, the rigid PCB R-PCB may be a traditional glass-reinforced multilayer circuit board, wherein the flexible PCB F-PCB is provided by a much thinner, flexible substrate on which traces and components may be formed or mounted. The flexible PCB F-PCB has a flex factor of at least ten times that of the rigid PCB R-PCB.

As illustrated, the control circuitry 44 and the RF circuitry 48 are implemented in whole or in part on the rigid PCB R-PCB while the MPMS filters 62, 66, 70 and the antenna tuning circuitry 50, 52, 54, 56 are implemented on the flexible PCB F-PCB. The coaxial cables 58, 60, 64, 68 connect the rigid PCB R-PCB and the flexible PCB F-PCB such that the transmit/receive paths that extend between the RF circuitry 48 and the respective antennas A1, A2, A3, and A4 are provided by the combination of the rigid PCB R-PCB, the flexible PCB F-PCB, and the coaxial cables 58, 60, 64, 68. These transmit/receive paths extend to corresponding antenna ports AP1, AP2, AP3, AP4 of the flexible PCB F-PCB. The antennas A1, A2, A3, and A4 are connected to the antenna ports AP1, AP2, AP3, AP4, respectively, through cables, traces, and/or the like.

With reference to FIG. 10, a low-noise amplifier (LNA) 72 may be provided between the MPMS filter 62 and the coaxial cable 60 to amplify the filtered receive signals prior to the coaxial cable 60. The LNA 72 may be provided along with the MPMS filter 62 on the flexible PCB F-PCB, wherein the RF circuitry 48 is provided on the rigid PCB R-PCB, and the coaxial cable 60 connects the flexible PCB F-PCB and the rigid PCB R-PCB. The antenna tuning circuitry 50, 52 may also be provided on the flexible PCB F-PCB.

The following provides various filters that employ acoustic resonators and are capable of providing a filter response that includes multiple passbands and stopbands. Some basics regarding the theory of operation are provided prior to describing the specific configurations, which provide the desired filter responses.

Turning now to FIG. 11, a series resonator B1 is shown coupled between an input node VP and an output node O/P. The series resonator B1 has a series resonance frequency F_(s) and inherent capacitance, which generally limits the bandwidth of filters that employ the series resonator B1. In the case of a Bulk Acoustic Wave (BAW) resonator, the capacitance of the series resonator B1 is primarily caused by its inherent structure, which looks and acts like a capacitor in part because the series resonator includes the top and bottom electrodes 20, 22 (FIG. 1) that are separated by a dielectric piezoelectric layer 18. While BAW resonators are the focus of this and the following examples, other types of acoustic resonators, such as Surface Acoustic Wave (SAW) resonators, are equally applicable.

A compensation circuit 74 is coupled in parallel with the series resonator B1 and functions to compensate for some of the capacitance presented by the series resonator B1. The compensation circuit 74 includes two negatively coupled inductors L1, L2 and a shunt resonator B2. The negatively coupled inductors L1, L2 are coupled in series between the input node I/P and the output node O/P, wherein a common node CN is provided between the negatively coupled inductors L1, L2. The negatively coupled inductors L1, L2 are magnetically coupled by a coupling factor K, wherein the dots illustrated in association with the negatively coupled inductors L1, L2 indicate that the magnetic coupling is negative. As such, the negatively coupled inductors L1, L2 are connected in electrical series and negatively coupled from a magnetic coupling perspective. As defined herein, two (or more) series-connected inductors that are negatively coupled from a magnetic perspective are inductors that are:

-   -   connected in electrical series; and     -   the mutual inductance between the two inductors functions to         decrease the total inductance of the two (or more) inductors.

The shunt resonator B2 is coupled between the common node CN and ground, or other fixed voltage node.

To compensate for at least some of the capacitance of the series resonator B1, the compensation circuit 74 presents itself as a negative capacitance within certain frequency ranges when coupled in parallel with the series resonator B1. Since capacitances in parallel are additive, providing a negative capacitance in parallel with the (positive) capacitance of the series resonator B1 effectively reduces the capacitance of the series resonator B1. With the compensation circuit 74, the series resonator B1 can actually function as a filter (instead of just a resonator) and provide a passband, albeit a fairly narrow passband, instead of a more traditional resonator response (solid line of FIG. 2).

FIG. 12 graphically illustrates the frequency responses of the series resonator B1 (inside the block referenced B1), the compensation circuit 74 (inside the block referenced 74), and the overall circuit in which the compensation circuit 74 is placed in parallel with the series resonator B1. As illustrated, the overall circuit provides a relatively narrow passband. Further detail on this particular circuit topology can be found in the co-assigned U.S. patent application Ser. No. 15/004,084, filed Jan. 22, 2016, now U.S. Pat. No. 9,837,984, and titled RF LADDER FILTER WITH SIMPLIFIED ACOUSTIC RF RESONATOR PARALLEL CAPACITANCE COMPENSATION, and U.S. patent application Ser. No. 14/757,651, filed Dec. 23, 2015, and titled SIMPLIFIED ACOUSTIC RF RESONATOR PARALLEL CAPACITANCE COMPENSATION, the disclosures of which are incorporated herein by reference in their entireties.

While beneficial in many applications, the narrow passband of the circuit topology of FIG. 11 has its limitations. With the challenges of modern day communication systems, wider passbands and the ability to provide multiple passbands within a given system are needed. Fortunately, applicants have discovered that certain modifications to this topology provide significant and truly unexpected increases in passband bandwidths and, in certain instances, the ability to generate multiple passbands of the same or varying bandwidths in an efficient and effective manner.

With reference to FIG. 13, a modified circuit topology is illustrated wherein the circuit topology of FIG. 11 is modified to include an additional shunt resonator B3, which is coupled between the common node CN and ground. As such, a new compensation circuit 76 is created that includes the negatively coupled inductors L1 and L2, which have a coupling coefficient K, and at least two shunt resonators B2, B3. The compensation circuit 76 is coupled in parallel with the series resonator B1. When the series resonance frequencies F_(s) of the shunt resonators B2, B3 are different from one another, unexpectedly wide bandwidths passbands are achievable while maintaining a very flat passbands, steep skirts, and excellent cancellation of signals outside of the passbands.

FIG. 14 graphically illustrates the frequency responses of the series resonator B1 (inside the block referenced B1), the compensation circuit 76 (inside the block referenced 76), and the overall circuit in which the compensation circuit 76 is placed in parallel with the series resonator B1. As illustrated, the overall circuit with the compensation circuit 76 provides a much wider passband (FIG. 10) than the overall circuitry with the compensation circuit 74 (FIG. 12).

While FIGS. 12 and 14 are graphical representations, FIG. 15 is an actual comparison of the frequency response of the overall circuit using the different compensation circuits 74, 76, wherein the overall circuit using the compensation circuit 76 provides a significantly wider and better formed passband (solid line) than the overall circuit using the compensation circuit 74 (dashed line).

As illustrated in FIG. 16, the concepts described herein not only contemplate the use of multiple shunt resonators B2, B3, which are coupled between the common node CN and ground, but also multiple series resonators, such as series resonators B1 and B4, which are coupled in parallel with one another between the input node VP and the output node O/P. The series resonance frequencies F_(s) of the series resonators B1, B4 are different from one another, and the series resonance frequencies F_(s) of the shunt resonators B2, B3 are also different from one another and different from those of the series resonators. While only two series resonators B1, B4 and two shunt resonators B2, B3 are illustrated, any number of these resonators may be employed depending on the application and the desired characteristics of the overall frequency response of the circuit in which these resonators and associated compensation circuits 76 are employed. While the theory of operation is described further subsequently, FIGS. 17 and 18 illustrate just two of the many possibilities.

For FIG. 17, there are two series resonators B1, B4 and two shunt resonators B2, B3, with different and relatively dispersed series resonance frequencies F_(s). FIG. 17 graphically illustrates the frequency responses of the combination of the two series resonators B1, B4 (inside the block referenced BX), the compensation circuit 76 with two shunt resonators B2, B3 (inside the block referenced 76), and the overall circuit in which the compensation circuit 76 is placed in parallel with the series resonator B1. As illustrated, the overall circuit in this configuration has the potential to provide a passband that is even wider than that for the embodiment of FIGS. 13 and 14. For example, passbands of greater than 100 MHz, 150 MHz, 175 MHz, and 200 MHz are contemplated at frequencies at or above 1.5 GHz, 1.75 GHz, and 2 GHz.

In other words, center-frequency-to-bandwidth ratios (fc/BW*100) of 3.5% to 9%, 12%, or greater are possible, wherein fc is the center frequency of the passband and BW is the bandwidth of the passband. If multiple passbands are provided, BW may encompass all of the provided passbands. Further, when multiple passbands are provided, the passbands may have the same or different bandwidths or center-frequency-to-bandwidth ratios. For example, one passband may have a relatively large center-frequency-to-bandwidth ratio, such as 12%, and a second passband may have a relatively small center-frequency-to-bandwidth ratio, such as 2%. Alternatively, multiple ones of the passbands may have a bandwidth of 100 MHz, or multiple ones of the passbands may have generally the same center-frequency-to-bandwidth ratios. In the latter case, the bandwidths of the passbands may inherently be different from one another, even though the center-frequency-to-bandwidth ratios are the same.

For FIG. 18, there are four series resonators, which are coupled in parallel with one another (not shown), and two shunt resonators (not shown) with different and more widely dispersed series resonance frequencies F_(s). FIG. 18 graphically illustrates the frequency responses of the combination of the four series resonators (inside the block referenced BX), the compensation circuit 76 with two shunt resonators B2, B3 (inside the block referenced 76), and the overall circuit in which the compensation circuit 76 is placed in parallel with the series resonator B1. As illustrated, the overall circuit in this configuration provides multiple passbands, which are separated by a stopband. In this embodiment, two passbands are provided; however, the number of passbands may exceed two. The number of passbands in the bandwidth of each of the passbands is a function of the number of shunt and series resonators B1-B4 and the series resonance frequencies F_(s) thereof.

The theory of the compensation circuit 76 follows and is described in association with FIGS. 19A through 19D and FIG. 20. With reference to FIG. 19A, assume the compensation circuit 76 includes the two negatively coupled inductors L1, L2, which have an inductance value L, and two or more shunt resonators BY, which have an overall shunt impedance Zres presented between the common node CN and ground. While the inductance values L of the negatively coupled inductors L1, L2 are described as being the same, these values may differ depending on the application. Also assume that the one or more series resonators BX present an overall series impedance ZS.

As shown in FIG. 19B, the two negatively coupled and series-connected inductors L1, L2 (without Zres) can be modeled as a T-network of three inductors L3, L4, and L5, wherein series inductors L3 and L4 are connected in series and have a value of L(1+K), and shunt inductor L5 has a value of −L*K, where K is a coupling factor between the negatively coupled inductors L1, L2. Notably, the coupling factor K is a positive number between 0 and 1. Based on this model, the overall impedance of the compensation circuit 76 is modeled as illustrated in FIG. 19C, wherein the shunt impedance Zres is coupled between the shunt inductor L5 and ground. The resulting T-network, as illustrated in FIG. 19C, can be transformed into an equivalent π (pi) network, as illustrated in FIG. 19D.

The π network of FIG. 19D can be broken into a series equivalent impedance ZA and two shunt equivalent impedances ZB. The series equivalent impedance ZA is represented by two series inductances of value L*(1+K), where K>0, and a special “inversion” impedance Zinv. The inversion impedance Zinv is equal to [L(1+K)ω]²/[Zres−jLKω], where ω=2πf and f is the frequency. As such, the series equivalent impedance ZA equals j*2*L(1+K)ω+Zinv and is coupled between the input node I/O and the output node O/P. Each of the two shunt equivalent impedances ZB is represented by an inductor of value L(1−K) in series with two overall shunt impedances Zres.

Notably, the series equivalent impedance ZA has a negative capacitor behavior at certain frequencies at which broadband cancellation is desired and has series resonance at multiple frequencies. In general, the series equivalent impedance ZA has a multiple bandpass-bandstop characteristic, in that the series equivalent impedance ZA passes some frequencies and stops others. When the series equivalent impedance ZA is placed in parallel with the series impedance ZS of the series resonators BX, which can also have a multiple bandpass-bandstop characteristic, a broadband filter or a filter with multiple passbands may be created.

FIG. 20 illustrates the series impedance ZS of the series resonators BX in parallel with the series equivalent impedance ZA of the compensation circuit 76. The overall series impedance ZAs represents the series impedance ZS in parallel with the series equivalent impedance ZA. The two shunt equivalent impedances ZB are respectively coupled between the input port I/P and ground and the output port O/P and ground. The primary focus for the following discussion relates to the series equivalent impedance ZA and its impact on the series impedance ZS when the series equivalent impedance ZA is placed in parallel with the series impedance ZS.

As noted previously, the series equivalent impedance ZA provides two primary functions. The first provides a negative capacitive behavior, and the second provides one or more additional series resonances between the input node I/P and the output node O/P. These additional series resonances are provided through the series equivalent impedance ZA and are in addition to any series resonances that are provided through the series impedance ZS of the series resonators BX. To help explain the benefits and concept of the negative capacitive behavior provided by the series equivalent impedance ZA, normal capacitive behavior is illustrated in association with the overall shunt impedance Zres, which is provided by the shunt resonators BY. FIG. 21 graphs the absolute (magnitude) and imaginary components of the overall shunt impedance Zres, which is formed by two shunt resonators BY coupled in parallel with one another.

The series resonance frequency F_(s) for each of the two shunt resonators BY occurs when the absolute impedance (abs(Zres)) is at or near zero. Since there are two shunt resonators BY, the absolute impedance (abs(Zres)) is at or near zero at two frequencies, and as such, there are two series resonance frequencies F_(s). The parallel resonance frequencies F_(p) occur when the imaginary component (imag(Zres)) peaks. Again, since there are two shunt resonators BY, there are two series resonance frequencies F_(s) provided by the overall shunt impedance Zres.

Whenever the imaginary component (imag(Zres)) of the overall shunt impedance Zres is less than zero, the overall shunt impedance Zres has a capacitive behavior. The capacitive behavior is characterized in that the reactance of the overall shunt impedance Zres is negative and decreases as frequency increases, which is consistent with capacitive reactance, which is represented by 1/jωC. The graph of FIG. 21 identifies three regions within the impedance response of the overall shunt impedance Zres that exhibit capacitive behavior.

Turning now to FIG. 22, the series equivalent impedance ZA is illustrated over the same frequency range as that of the overall shunt impedance Zres, illustrated in FIG. 21. The series equivalent impedance ZA has two series resonance frequencies F_(s), which occur when the absolute impedance (abs(ZA)) is at or near zero. The two series resonance frequencies F_(s) for the series equivalent impedance ZA are different from each other and slightly different from those for the overall shunt impedance Zres. Further, the number of series resonance frequencies F_(s) generally corresponds to the number of shunt resonators BY in the compensation circuit 76, assuming the series resonance frequencies F_(s) are different from one another.

Interestingly, the imaginary component (imag(ZA)) of the series equivalent impedance ZA is somewhat inverted with respect to that of the overall shunt impedance Zres. Further, the imaginary component (imag(ZA)) of the series equivalent impedance ZA has a predominantly positive reactance. During the portions at which the imaginary component (imag(ZA)) is positive, the reactance of the series equivalent impedance ZA again decreases as frequency increases, which is indicative of capacitive behavior. However, the reactance is positive, whereas traditional capacitive behavior presents a negative reactance. This phenomenon is referred to as negative capacitive behavior. Those portions of the imaginary component (imag(ZA)) of the series equivalent impedance ZA that are positive and thus exhibit negative capacitive behavior are highlighted in the graph of FIG. 22.

The negative capacitive behavior of the series equivalent impedance ZA for the compensation circuit 76 is important, because when the series equivalent impedance ZA is placed in parallel with the series impedance ZS, the effective capacitance of the overall circuit is reduced. Reducing the effective capacitance of the overall circuit shifts the parallel resonance frequency F_(p) of the series impedance ZS higher in the frequency range, which is described subsequently, and significantly increases the available bandwidth for passbands while providing excellent out-of-band rejection.

An example of the benefit is illustrated in FIGS. 23A and 23B. The thicker solid line, which is labeled abs(VG), represents the frequency response of the overall circuit illustrated in FIG. 16, wherein there are two series resonators BX and two shunt resonators BY in the compensation circuit 76. The frequency response has two well-defined passbands, which are separated by a stopband. The frequency response abs(VG) of the overall circuit generally corresponds to the inverse of the overall series impedance ZAs, which again represents the series impedance Zs in parallel with the series equivalent impedance ZA, as provided in FIG. 20.

Notably, the parallel resonance frequencies F_(p)(ZS) of the series impedance ZS, in isolation, fall in the middle of the passbands of frequency response abs(VG) of the overall circuit. If the parallel resonance frequencies F_(p)(ZS) of the series impedance ZS remained at these locations, the passbands would be severely affected. However, the negative capacitive behavior of the series equivalent impedance ZA functions to shift these parallel resonance frequencies F_(p)(ZS) of the series impedance ZS to a higher frequency and, in this instance, above the respective passbands. This is manifested in the resulting overall series impedance ZAs, in which the only parallel resonance frequencies F_(p)(ZAs) occur above and outside of the respective passbands. An additional benefit to having the parallel resonance frequencies F_(p)(ZAs) occur outside of the respective passbands is the additional cancellation of frequencies outside of the passbands. Plus, the overall series impedance ZAs is lower than the series impedance ZS within the respective passbands.

A further contributor to the exemplary frequency response abs(VG) of the overall circuit is the presence of the additional series resonance frequencies F_(s), which are provided through the series equivalent impedance ZA. These series resonance frequencies F_(s) are offset from each other and from those provided through the series impedance ZS. The series resonance frequencies F_(s) for the series equivalent impedance ZA in the series impedance ZS occur when the magnitudes of the respective impedances approach zero. The practical results are wider passbands, steeper skirts for the passbands, and greater rejection outside of the passbands, as evidenced by the frequency response abs(VG) of the overall circuit.

Turning now to FIG. 24, an alternative filter circuit that employs inductive coupling is provided. In essence, two filter paths P1, P2 extend between an input node I/P and an output node O/P. The first filter path P1 includes two inductors L11, L12, which are positively coupled with one another and connected in series between the input node I/P and the output node O/P. The coupling factor Ka between the inductor L11 and inductor L12 is between zero and one. The inductors L11, L12 are connected at a common node CN10. A plurality of acoustic resonators B11 through B13 are coupled in parallel with one another between the common node CN10 and ground (or other fixed voltage node). While only three acoustic resonators B11 through B13 are shown, any number of acoustic resonators may be employed.

The second filter path P2 is similarly configured to that of the first filter path P1, and includes two inductors L21, L22, which are negatively coupled with one another and connected in series between the input node I/P and the output node O/P. The coupling factor Ks between the inductor L21 and inductor L22 is between zero and one and may be the same or different from that of the coupling factor Ka. The inductors L21, L22 are connected at a common node CN20. A plurality of acoustic resonators B21 through B23 are coupled in parallel with one another between the common node CN20 and ground. While only three acoustic resonators B21 through B23 are shown, any number of acoustic resonators may be employed.

The response of the filter circuit may be provide bandpass, notch (stopband), or multiple bandpass/stopband responses, depending on the selection of series resonance frequencies f11, f12, f13 for the acoustic resonators B11, B12, B13 of the first filter path P1 and the series resonance frequencies f21, f22, f23 of the acoustic resonators B21, B22, B23 of the second filter path P2. In one embodiment, the series resonance frequencies f11, f12, f13, f21, f22, f23 for the acoustic resonators B11, B12, B13, B21, B22, B23 are different from one another and are located to provide the desired filter response.

Turning now to FIG. 25, another filter circuit F1 that employs inductive coupling is provided. Again, the filter circuit F1 extends between an input node I/P and an output node O/P. The filter circuit F1 has a first path P3 that extends between the input node I/P and ground (or other fixed voltage node). The first path P3 includes two or more acoustic resonators B31, B32, which are coupled in parallel with one another and in series with an inductor L31. The filter circuit F1 has a second path P4 that extends between the output node O/P and ground (or other fixed voltage node). The second path P4 includes two or more acoustic resonators B41, B42, which are coupled in parallel with one another and in series with an inductor L32. The inductors L31, L32 are negatively coupled and have a coupling factor of K. In one embodiment, the acoustic resonators B31, B32 of the first path P3 have series resonance frequencies that differ from one another, and the acoustic resonators B41, B42 have series resonance frequencies that differ from one another. However, each acoustic resonator B31, B32 of the first path P3 may have the same series resonance frequency as that of one of the acoustic resonators B41, B42 of the second path P4. For example, acoustic resonators B31, B41 may have a first series resonance frequency, and acoustic resonators B32, B42 may have a second series resonance frequency, which is different from the first series resonance frequency. The series resonance frequencies of the various acoustic resonators B31, B32, B41, and B42 may also be different from one another in alternative embodiments. As with the embodiment of FIG. 24, the series resonance frequencies chosen for the acoustic resonators B31, B32, B41, and B42 in the first and second paths P3, P4 dictates the type of filter response for the filter circuit F1, which may include bandpass, notch (stopband), or multiple bandpass/stopband responses.

As illustrated in FIG. 26, a variant of the filter circuit F1 of FIG. 25 may be placed in parallel with the filter circuit F1 in order to provide a combined filter circuit. In essence, the filter circuit F1 is placed in parallel with filter circuit F2, wherein both filter circuits F1, F2 extend between the input node I/P and the output node O/P.

Notably, filter circuit F2 employs positive inductive coupling as opposed to the negative inductive coupling of filter circuit F1. The filter circuit F2 has a first path P3′ that extends between the input node I/P and ground (or other fixed voltage node). The first path P3′ includes two or more acoustic resonators B31′, B32′, which are coupled in parallel with one another and in series with an inductor L31′. The filter circuit F2 has a second path P4′ that extends between the output node O/P and ground (or other fixed voltage node). The second path P4′ includes two or more acoustic resonators B41′, B42′, which are coupled in parallel with one another and in series with an inductor L32′. The inductors L31′, L32′ are positively coupled. In one embodiment, the acoustic resonators B31′, B32′ of the first path P3′ have series resonance frequencies that differ from one another, and the acoustic resonators B41′, B42′ have series resonance frequencies that differ from one another. However, each acoustic resonator B31′, B32′ of the first path P3′ may have the same series resonance frequency as that of one of the acoustic resonators B41′, B42′ of the second path P4′. For example, acoustic resonators B31′, B41′ may have a first series resonance frequency, and acoustic resonators B32′, B42′ may have a second series resonance frequency, which is different from the first series resonance frequency. The series resonance frequencies of the various acoustic resonators B31′, B32′, B41′, and B42′ may also be different from one another in alternative embodiments. In most embodiments, the respective acoustic resonators B31, B32, B41, and B42 of the first filter circuit F1 have series resonance frequencies that differ from the series resonance frequencies of the acoustic resonators B31′, B32′, B41′, and B42′ of the second filter circuit F2. While only two filter circuits F1, F2 are illustrated, additional filter circuits may be provided in parallel with these filter circuits F1, F2. Regardless of configuration, the overall filter circuit may provide bandpass, notch (stopband), or multiple bandpass/stopband responses, depending on the selection of resonance frequencies.

Yet another embodiment is illustrated in FIG. 27. The illustrated filter circuit is similar to that illustrated in FIG. 16. The difference is that an inductive and/or capacitive element 80 is placed in series with one or more of the acoustic resonators B2, B3 of the compensation circuit 76. Additionally, another inductive and/or capacitive element 82 may be placed in series with one or more of the series acoustic resonators B1, B4. The inductive and/or capacitive elements 80, 82 provide additional variables for adjusting the filter response of the filter circuit.

FIG. 28 provides an additional variant of the filter circuit of FIG. 16, wherein the acoustic resonators B2, B3 of the compensation circuit 76 are replaced with an LC circuit. In particular, a compensation circuit 76′ includes the two negatively coupled inductors L1, L2, as described previously and an inductor L′ and a capacitor C′, which are coupled in parallel with one another between the common node CN and ground (or other fixed voltage note). If the coupling factor K is equal to or approaches 1, extremely broadband passbands or stopbands, as well as multiple passbands and stopbands spread over a broad frequency range, are possible.

In particular (with concurrent reference to FIG. 16 and the description thereof), ZA=j*2*L*(1+K)*w−[L(1+K)*w]²/[Zres−j*L*K*w].

Zres can be expressed from the equation as function of ZA, such that:

Zres=[+j*L*K*w*ZA−L²w²*(1+K)*(1−K)]/[ZA−j*2*L*(1+K)*w], and

Zres=[+j*L*K*w*ZA−L²w²*(1−K²)]/[ZA−j*2*L*(1+K)*w].

If we take only the parallel C0 capacitor part of Zs (i.e., capacitance of series resonators B1, B4), Zs_C0=−j/(C0*w).

To synthesize a parallel ZA impedance that cancels C0:

ZA=−Zs_C0=+j/(C0*w).

Thus, we can replace ZA in the foregoing Zres equation, as follows:

Zres to create negative C0=[+j²*L*K*w/(C0*w)−L²w²*(1−K²)]/[+j/(C0*w−j*2*L*(1+K)*w];

Zres to create negative C0=[−LK/C0−L²w²*(1−K²)]/(+j*[+1/(C0*w)−2*L*(1+K)*w]);

Zres to create negative C0=+j[LK/C0+L²w²*(1−K²)]/[+1/(C0*w)−2*L*(1+K)*w].

As a result, Yres=1/Zres to create negative C0=−j*[+1/(C0*w)−2*L*(1+K)*w]/[K/C0+L²w²*(1−K²)].

If we have K=1 (ideal coupling), Yres for K=1=−j/(L*w)+j*4*C0*w.

Thus, Yres is made of two parallel admittances, an inductor L′ of value L and a capacitor C′ of value 4*C0 for the case of two negatively coupled inductors of L value with K=1. The result is exceptionally broad bandwidths from a single filter structure that has very low insertion loss.

FIG. 29 illustrates a unique implementation of the filter circuit illustrated in FIG. 16 and a differential amplifier circuit. The differential amplifier circuit includes differential power amplifiers 84, 86, which drive a primary (winding) side of a center-tapped transformer T1. The center tap of the primary side of transformer T1 is coupled to a supply voltage Vcc and to ground through a capacitor C″. The secondary (winding) side of the transformer T1 provides the negatively coupled inductors L1 and L2 for the compensation circuit 76. The acoustic resonators B2, B3 for the compensation circuit 76 are coupled between the center tap of the secondary side of the transformer T1 and ground (or other fixed voltage note). The series acoustic resonators B1, B4 of the compensation circuit 76 are effectively coupled across the secondary side of the transformer T1. In operation, a differential input signal (DIFF I/P) drives the differential power amplifiers 84, 86 and a single-ended output signal (O/P) is presented to a downstream circuit element, such as another ladder network 88.

Another application for the concepts disclosed herein is illustrated in FIG. 30. A transmit signal (TX signal) is presented to a power amplifier 90 and passed through a single, multiband filter 92 to an antenna node AN, which is coupled directly or indirectly to an antenna (not shown). The multiband filter 92 is configured according to one or more of the concepts described previously, and in this example, is configured to provide three distinct passbands, which are separated by stopbands and are centered about transmit band A, transmit band B, and transmit band C, respectively. Multiple receive filters 94, 96, 98 are also coupled to the antenna node AN and are used to separate the receive signals and receive band A, receive band B, and receive band C. As such, the single, multiband filter 92 may be used in the transmit path, while multiple receive filters 94, 96, 98 may be used in the receive path, and vice versa.

Another variant is illustrated in FIG. 31, wherein the transmit signal (TX signal) is presented to a first port of a first hybrid coupler 102. A second port of the first hybrid coupler 102 is coupled to ground (or other fixed voltage noted) through a resistor R_(C). The third and fourth ports of the first hybrid coupler 102 are respectively coupled to the inputs of two multiband filters 92, which provide multiple distinct passbands, separated by stop bands. As illustrated, the multiband filters 92 provide passbands about transmit band A, transmit band B, and transmit band C. The respective outputs of the multiband filters 92 drive first and second ports of a second hybrid coupler 104. The third port of the second hybrid coupler 104 is coupled to multiple receive filters 94, 96, 98, which are used to pass signals in receive band A, receive band B, and receive band C, respectively.

While the concepts disclosed herein are described in association with a mobile terminal, these concepts are applicable to any type of communication device that employs wireless communications. Those skilled in the art will recognize numerous modifications and other embodiments that incorporate the concepts described herein. These modifications and embodiments are considered to be within scope of the teachings provided herein and the claims that follow. 

What is claimed is:
 1. Filter circuit comprising: a first node and a second node; a first path coupled between the first node and the second node and comprising: a first inductor and a second inductor coupled in series between the first node and the second node, wherein the first inductor and the second inductor are positively coupled with one another and a first common node is provided between the first inductor and the second inductor; and a first plurality of shunt acoustic resonators coupled between the first common node and a fixed voltage node; a second path coupled between the first node and the second node and comprising: a third inductor and a fourth inductor coupled in series between the first node and the second node, wherein the third inductor and the fourth inductor are negatively coupled with one another and a second common node is provided between the third inductor and the fourth inductor; and a second plurality of shunt acoustic resonators coupled between the second common node and a fixed voltage node, wherein a filter response of one of a passband, a stopband, and a multiple passband/stopband is provided between the first node and the second node.
 2. The filter circuit of claim 1 wherein each of the first plurality of shunt acoustic resonators has a series resonance frequency that is different from the series resonance frequency of the other of the first plurality of shunt acoustic resonators.
 3. The filter circuit of claim 2 wherein each of the second plurality of shunt acoustic resonators has a series resonance frequency that is different from the series resonance frequency of the other of the second plurality of shunt acoustic resonators.
 4. The filter circuit of claim 1 wherein: each of the second plurality of shunt acoustic resonators has a series resonance frequency that is different from the series resonance frequency of the other of the second plurality of shunt acoustic resonators and from the series resonance frequency of each of the first plurality of shunt acoustic resonators; and each of the first plurality of shunt acoustic resonators has a series resonance frequency that is different from the series resonance frequency of the other of the first plurality of shunt acoustic resonators and from the series resonance frequency of each of the second plurality of shunt acoustic resonators.
 5. The filter circuit of claim 1 wherein the filter response is the multiple passband/stopband.
 6. The filter circuit of claim 1 wherein a first coupling coefficient between the first inductor and the second inductor is different from a second coupling coefficient between the third inductor and the fourth inductor.
 7. The filter circuit of claim 1 wherein the first plurality of shunt acoustic resonators and the second plurality of shunt acoustic resonators are bulk acoustic wave resonators.
 8. Filter circuit comprising: an input node, a first node, a second node, an output node, and a fixed voltage node; a first plurality of acoustic resonators that are coupled in parallel with one another between the input node and the first node; a first inductor coupled between the first node and the fixed voltage node such that the first plurality of acoustic resonators is coupled in series with the first inductor between the input node and the fixed voltage node; a second plurality of acoustic resonators that are coupled in parallel with one another between the second node and the output node; and a second inductor coupled between the second node and the fixed voltage node such that the second plurality of acoustic resonators is coupled in series with the second inductor between the output node and the fixed voltage node, wherein the first inductor and the second inductor are negatively coupled with one another.
 9. The filter circuit of claim 8 wherein each of the first plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that is different from the series resonance frequency of the other of the first plurality of acoustic resonators that are coupled in parallel with one another.
 10. The filter circuit of claim 9 wherein each of the second plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that corresponds to one of the first plurality of acoustic resonators that are coupled in parallel with one another.
 11. The filter circuit of claim 8 wherein a filter response of one of a passband, a stopband, and a multiple passband/stopband is provided between the first node and the second node.
 12. The filter circuit of claim 8 further comprising: a third node and a fourth node; a third plurality of acoustic resonators that are coupled in parallel with one another between the input node and the third node; a third inductor coupled between the third node and the fixed voltage node such that the third plurality of acoustic resonators is coupled in series with the third inductor between the input node and the fixed voltage node; a fourth plurality of acoustic resonators that are coupled in parallel with one another between the fourth node and the output node; and a fourth inductor coupled between the fourth node and the fixed voltage node such that the fourth plurality of acoustic resonators is coupled in series with the fourth inductor between the output node and the fixed voltage node, wherein the third inductor and the fourth inductor are positively coupled with one another.
 13. The filter circuit of claim 12 wherein each of the first plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that is different from the series resonance frequency of the other of the first plurality of acoustic resonators that are coupled in parallel with one another.
 14. The filter circuit of claim 13 wherein each of the second plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that corresponds to one of the first plurality of acoustic resonators that are coupled in parallel with one another.
 15. The filter circuit of claim 14 wherein: each of the third plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that is different from the series resonance frequency of the other of the third plurality of acoustic resonators that are coupled in parallel with one another; each of the fourth plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that corresponds to one of the third plurality of acoustic resonators that are coupled in parallel with one another.
 16. The filter circuit of claim 15 wherein each of the first and second plurality of acoustic resonators that are coupled in parallel with one another has a series resonance frequency that is different from the series resonance frequency of each of the third and fourth plurality of acoustic resonators that are coupled in parallel with one another.
 17. The filter circuit of claim 16 wherein a filter response of one of a passband, a stopband, and a multiple passband/stopband is provided between the first node and the second node.
 18. The filter circuitry of claim 12 wherein a filter response of one of a passband, a stopband, and a multiple passband/stopband is provided between the first node and the second node.
 19. A differential power amplifier circuit comprising: a first power amplifier having a first input node and a first output node; a second power amplifier having a second input node and a second output node; a first transformer comprising: a primary side coupled between the first output node and the second output node; and a secondary side formed from a first inductor and a second inductor coupled in series between a first node and a second node wherein a common node is provided between the first inductor and the second inductor; at least one series acoustic resonator coupled between the first node and the second node wherein at least one main series resonance is provided between the first node and the second node at a main resonance frequency through the at least one series acoustic resonator; a first shunt acoustic resonator coupled between the common node and a fixed voltage node; and a second shunt acoustic resonator coupled between the common node and the fixed voltage node, wherein a differential input is provided across the first input node and the second input node, and a single-ended output signal is provided at the second node.
 20. The differential amplifier circuit of claim 19 wherein a first series resonance at a first resonance frequency and a second series resonance at a second resonance frequency, which is different from the first resonance frequency and the main resonance frequency, are provided between the first node and the second node.
 21. The differential amplifier circuit of claim 20 wherein at least one of the first resonance frequency and the second resonance frequency is greater than the main resonance frequency.
 22. The differential amplifier circuit of claim 20 wherein the first resonance frequency is less than the main resonance frequency, and the second resonance frequency is greater than the main resonance frequency.
 23. The differential amplifier circuit of claim 19 wherein the first inductor and the second inductor are negatively coupled with one another.
 24. The differential amplifier circuit of claim 19 wherein the at least one series acoustic resonator comprises a plurality of acoustic resonators that are coupled in parallel with one another and each of the plurality of acoustic resonators has a different series resonance frequency.
 25. The differential amplifier circuit of claim 19 further comprising at least one additional shunt acoustic resonator coupled between the common node and the fixed voltage node. 